Class-D amplifiers are electronic amplifiers in which the amplifying device operates as an electronic switch, instead of as linear gain devices as in other amplifiers. In class-D amplifiers, the analog signal is amplified using a system of switches that convert the signal into a series of pulses by pulse width modulation, pulse density modulation or another method. The pulses that make up the amplified signals are then passed through a low pass filter that filters out the high frequency signal components to form an amplified version of the original signal. The filter is constructed from capacitive and inductive elements so that the energy losses associated with the filter are low. Class-D amplifiers are designed to track the input signal in real-time, and usually work in the frequency range of up to tens of kHz, making them suitable for amplifying audio signals.
The efficiency of class-D amplifiers is above 90%. This is because the amplified signal is binary and has segments in which the current passing through the switch is zero and the voltage finite, and segments in which the current through the switch is finite and the voltage is zero. In an electric circuit, heat loss is equal to the product of current and voltage. Therefore, in a system in which either the voltage or current are equal to zero most of the time, heat losses will be minimal. Class D amplifiers are therefore highly efficient amplifiers with minimal losses.
Using TTFields therapy to treat tumors is described in U.S. Pat. No. 7,805,201, and TTFields therapy requires the generation of a high voltage sinusoidal signal. Previously, generating this sinusoidal signal was implemented by generating a low amplitude signal with a function generator, then amplifying this signal using a linear amplifier and subsequently applying the signal to the electrodes that are positioned on the patient's body. The use of linear amplifier results in heat losses of close to 50%, reducing battery life, and complicating device design because a cooling system is required to dissipate the heat generated.
Implementing a high-efficiency digital signal generation/amplification architecture for TTFields would be beneficial. One possible approach for building such a system could be to replace the linear amplifier in the system with a Class-D amplifier. However, existing Class-D amplifier technology is not suitable for this task for two reasons. First, the signal distortion associated with Class-D amplifiers increases as the signal amplitude decreases, and TTFields requires sinewaves with very low levels of distortion at all signal levels. And second, TTFields therapy requires the generation of sinewaves at frequencies greater than 100 kHz. This requires very fast switches and complex control that are currently unavailable.